For decades there has been a constant search for methods, techniques, and equipment for the generation of Electricity. Although there have been significant advances in large scale energy generation, there has been little success in converting Thermal Energy (heat) to Electricity, especially solutions capable of operating on small temperature differences. Furthermore, current large scale Electricity generation systems discard significant Thermal Energy that could be used to produce additional Electricity, increasing the overall efficiency of the system.
The Stirling Engine, developed in 1816, is one method of generating Electricity from Thermal Energy. The Stirling Engine can achieve a rather high efficiency, up to 80% of the Carnot efficiency, limited only by non-ideal properties of the working gas and engine materials.
Stirling Engines use a closed cycle system having a trapped gas, “working fluid,” commonly air, hydrogen or helium. The engine is sealed and the engine passes through a cooling phase, a compression phase, a heating phase, and an expansion phase. The engine moves through the various phases as the “working fluid” is heated by an external heat source and cooled by a cold source, commonly a heat exchanger. Changes in gas temperature causes a corresponding change in gas pressure, while the motion of the engine causes gas to be alternately expanded and compressed. The motion produced by the engine can be attached to an Electric Generator to produce Electricity.
Therefore, the greater the temperature difference between the hot and cold sources, the greater the power output of the Stirling Engine. Stirling Engines can run on low temperature differences, for example, the difference between room temperature and a human hand can be used by a Stirling Engine to produce about 1 watt. However, these systems are large, heavy and inefficient for the miniscule power they produce.
Unfortunately, Stirling Engines require both input and output heat exchangers designed to contain the pressure of the working fluid at high temperatures while resisting corrosive effects of the heat source and of the atmosphere. Furthermore, at low temperature differences between the hot and cold sources, Stirling Engines do not produce enough energy to justify their cost, weight and size. Stirling Engines also have a number of disadvantages such as the need to warm up, creating a delayed start, and the necessity for an Electric Generator to convert the generated mechanical energy into Electricity, thereby increasing the size of the system, while decreasing efficiency.
Turbines are commonly used to extract Thermal Energy from a pressurized steam converting it to Mechanical Energy. The Mechanical Energy commonly drives an Electric Generator to produce Electricity. Although these systems are commonly used, they generally rely on high pressure, high temperature steam, which cannot be produced from small thermal sources, or small temperature differences. While efficient for larger energy production, Turbines are not suitable for small scale energy production.
Stirling Engines and Turbines convert Thermal Energy into Mechanical Energy, which, unfortunately, must go through another conversion via an Electric Generator or similar device to convert the Mechanical Energy into Electricity.
Electric Generators convert Mechanical Energy into Electricity using electromagnetic induction. Electric Generators use mechanical energy to rotate a coil about a permanent or powered magnet. Smaller Electric Generators, using permanent magnets, are generally about 40% efficient. With smaller, more powerful rare-earth magnets, efficiency can be increase to about 60%.
More efficient Electric Generators, using a field coil in place of a permanent magnet to generate a magnetic field, can achieve efficiencies of up to 90%. A battery is generally used to power the field coil before a sufficient amount of Electricity is generated by the Electric Generator itself. This external source makes field coil based Electric Generators bulky and unsuitable for smaller designs.
Unfortunately, Electric Generators decrease the efficiency of a system while also adding weight and bulk, for example the weight and bulk of a permanent magnet or field coil.
Magnetohydrodyamic (MHD) generators are another method of transforming Mechanical Energy from Stirling Engine or Turbine based systems into Electricity. Generally in MHD systems, a conductive fluid is pumped by Mechanical Energy in the presence of a magnetic field. Similar to the conductive winding of an Electric Generator, a current is induced in the liquid, generating Electricity. Typically, MHD systems are about 17% efficient, making MHD systems undesirable for larger scale energy production.
Other systems similar to the one described in U.S. Pat. No. 4,191,901, hereby fully incorporated by reference, utilize a MHD design using the thermal expansion properties of a liquid. In these systems, thermal energy is applied to a sealed liquid. As the liquid is heated by thermal energy it is driven by free convection and moved a reduction in density of the heated liquid, which pushes the liquid through the system. Unfortunately, these systems have very poor efficiencies and result in an over complicated, over-sized and heavy solution for the energy they produce.
U.S. Pat. Nos. 2,510,397; 2,881,384; and 2,915,652, hereby fully incorporated by reference, describe devices capable of generating Electricity from temperature differences from thermionic emission within solid state devices. These devices commonly use two semiconductors, an anode and a cathode. The cathode is constructed of a material having a higher work function than the anode. Application of sufficient heat to the cathode causes thermionic emission, causing a small percentage of emitted electrons to reach the anode (moving from the higher work function of the cathode to the lower work function of the anode). Unfortunately, these systems are plagued by low power generation, and poor efficiency.
Generally, electrons freed by thermal energy at the cathode are motivated to traverse to the anode only by the different work functions. The presence of a cloud of free electrons in the anode, tend to repel electrons emitted from the cathode, known as the Space Charge Effect. This phenomenon limits electron movement between the cathode and anode to only those having a high enough velocity to pass through the Space Charge, significantly reducing efficiently. Although small, these systems have been unable to achieve any sizable power generation and are too cost prohibitive given the power they produce.
The foregoing solutions utilize substantially mature technologies. It is desirable to have a new method of generating electricity from thermal energy capable of maturing into methods and devices having properties unachievable by current methods.
Therefore it is desirable to generate Electricity in a compact, efficient manner from small temperature differences. Furthermore, it is desirable to have a system capable of directly producing Electricity without the need of an Electronic Generator, adding weight, size, while reducing efficiency.